1. Field of the Invention
The present invention relates to thin-film magnetic heads provided with a coil layer formed between core layers. In particular, the present invention relates to a thin-film magnetic head which has an upper core layer having an improved shape and can meet trends towards narrow tracks, and relates to a method for making the same.
Also, the present invention relates to a thin-film magnetic head which is provided with a coil layer formed between core layers and has satisfactory NLTS (nonlinear transition shift) characteristics and OW (overwrite) characteristics.
2. Description of the Related Art
FIG. 10 is a longitudinal cross-sectional view of a conventional thin-film magnetic head. The left end of the thin-film magnetic head in the drawing faces recording media. This thin-film magnetic head is a so-called "composite thin-film magnetic head" having a reading head h1 using a magnetoresistive effect and an inductive head h2 for writing signals onto recording media, such as a hard disk. The inductive head h2 is deposited on the reading head h1. This thin-film magnetic head is provided at the end of the trailing side of a slider of a floating-type magnetic head.
The reading head h1 has a lower core layer 1 composed of a magnetic material having high permeability, for example, a Fe--Ni alloy (permalloy). The lower core layer 1 also functions as an upper shielding layer of the reading head h1 by means of the magnetoresistive effect.
A gap layer 2 composed of a nonmagnetic material, such as alumina (Al.sub.2 O.sub.3), is provided on the lower core layer 1. As shown in FIG. 10, an insulating layer 3 composed of a resist or an organic resin is formed on the gap layer 2. Furthermore, a spiral coil layer 4 composed of a highly conductive material such as copper is formed on the insulating layer 3 so as to surround a base section 6b of an upper core layer 6. In FIG. 10, the coil layer 4 can be partially seen.
An insulating layer 5 composed of a resist or an organic resin is formed on the coil layer 4. The upper core layer 6 is formed by plating a magnetic material such as permalloy on the insulating layer 5. A front end section 6a of the upper core layer 6 is bonded to the lower core layer 1 with the gap layer 2 provided therebetween to form a magnetic gap having a gap length Gl.sub.1. The base section 6b of the upper core layer 6 is magnetically coupled with the lower core layer 1 through cavities formed in the gap layer 2 and the insulating layer 3.
FIG. 11 is a plan view of the thin-film magnetic head shown in FIG. 10. The upper core layer 6 consists of a leading region A having a constant width and a trailing region B gradually spreading from the leading end region. The leading region A of the upper core layer 6 is slender and has a width which is equal to the track width T.sub.W.
In the inductive head h2 for writing, recording currents flowing in the coil layer 4 induce recording magnetic fields in the lower core layer 1 and the upper core layer 6. Leakage magnetic fields from the magnetic gap section between the lower core layer 1 and the front end section 6a of the upper core layer 6 record magnetic signals on recording media, such as a hard disk.
As shown in FIG. 10, the reading head h1 includes a lower shielding layer 7 composed of a magnetic material, a magnetoresistive element layer 9 formed on the lower shielding layer 7 with a lower gap layer 8 provided therebetween 8, and an upper shielding layer or lower core layer 1 formed on the magnetoresistive element layer 9 with an upper gap layer 10 provided therebetween.
NLTS characteristics and OW characteristics, as important recording characteristics, greatly depend on the shape of the leading region A of the upper core layer 6. Herein, the NLTS (nonlinear transition shift) characteristics mean the phase lead caused by non-linear distortion of the leakage magnetic field, generated in the magnetic gap between the upper core layers 1 and the lower core layer 6 of the inductive head h2 in FIG. 10 by the leakage magnetic field from the magnetic recording signals which are just recorded on a recording medium towards the head.
The OW (overwrite) characteristics mean a difference in output of recorded signals at a low frequency between the initial output before overwriting at a high frequency and the decreased output after the overwriting.
When the leading region A of the upper core layer 6 is slender, as shown in FIG. 11, the length L1 of the leading region A is considered to be preferably in a range of approximately 4 .mu.m to 10 .mu.m. Such a length L1, however, causes deterioration of OW characteristics although it contributes to improvement in NLTS characteristics.
The upper core layer 6 of the thin-film magnetic head is formed by a frame plating process, as shown in FIG. 12. In this process, the coil layer 4 is formed and is covered with the insulating layer 5. An underlying layer 7 composed of a magnetic material such as a NiFe alloy is formed over the gap layer 2, exposed in the vicinity of the end, and the insulating layer 5. After a resist layer 8 is formed on the underlying layer 7, the resist layer 8 is exposed and developed to form a pattern of the shape of the upper core layer 6. A magnetic layer (upper core layer 6) is plated on the exposed underlying layer 7. The residual resist layer 8 is removed to form the upper core layer 6. The thin-film composite is cut along line Z--Z in FIG. 12 to form the thin-film magnetic head shown in FIG. 10, wherein the cut surface along line Z--Z faces the recording media.
Production of the upper core layer 6 of a conventional thin-film magnetic head has the following problems. FIG. 13 is a plan view when a resist layer 38 is formed on the underlying layer 7 and a pattern 39 of the upper core layer 6 is formed on the resist layer 38. As shown in FIG. 13, the pattern 39 of the upper core layer 6 consists of the leading region A having a track width T.sub.W at the left side in the drawing and the trailing region B spreading towards the right side. The current track width T.sub.W, approximately 2 to 3 .mu.m, of the leading region A must be decreased to 1 .mu.m or less in order to meet current high-density recording trends.
The pattern 39 is formed by exposing and developing the resist layer 38. In conventional processes, the slender leading region A and the spreading trailing region B are simultaneously exposed using short-wavelength light (g-line to i-line) at a low NA (numerical aperture) of 0.2 to 0.3 for achieving a large depth of focus. For example, the upper limit of the resolution is 1.0 .mu.m for a combination of the i-line and a numerical aperture NA of 0.2 to 0.3. Thus, a track width T.sub.W of less than 1.0 .mu.m is not achieved in conventional processes.
When the resist layer 38 on the leading region A is removed in the development step after the formation of the pattern 39 for the upper core layer 6, a developing solution barely penetrates into the resist layer 38 in the pattern 39 for the slender leading region A having a small width (track width T.sub.W). Thus, the resist layer 38 in the leading region A may be not completely removed. Accordingly, this process is not applicable to an upper core layer having a smaller track width T.sub.W.
In addition, the leading region A of the resulting thin-film magnetic head has a small track width T.sub.W and a large length L.sub.1. Thus, OW characteristics are decreased by damping of the magnetic flux density in the leading region A. It is known that the OW characteristics of the upper core layer 6 having the leading region A with the track width T.sub.W decrease as the length L1 of the leading region A increases.